We propose a comparative study of different heme proteins exhibiting monooxygenase, oxidase or peroxidase activity. By characterizing the electronic structure of the heme active site, specifically of the central iron, in all the stable reaction intermediates available, we hope to elucidate the factors controlling the diverse reactivities of these proteins. Our main probe, Mossbauer spectroscopy, is sensitive to the heme iron only and reveals its interactions with the immediate ligands and with nearby paramagnetic centers. Complementary EPR studies are carried out whenever possible. Specifically, the following systems are investigated: 1) Cytochrome P450 from the camphor hydroxylase of Pseudomonas putida. This system is studied in several reaction intermediates, and single crystal experiments are possible. 2) We will extend our analysis of the higher charge states, compounds I and II, of horseradish peroxidase to related compounds. 3) Nitrite reductase/cytochrome oxidase and cytochrome c551 from Pseudomonas aeruginosa. Here the possible interactions between heme c and d of the oxidase and between the oxidase and cytochrome c551 are of prime interest, and it is planned to study them with differentially enriched proteins.